Optical coherence tomography (OCT) provides cross-sectional images of the retina with exquisite axial resolution, and is commonly used in ophthalmology. High resolution OCT retinal imaging is important to non-invasively visualize the various retinal structures to aid in better understanding of the pathogenesis of vision-robbing diseases. In OCT, the axial resolution is determined by the optical spectrum of the light source, whereas the lateral resolution is determined by the numerical aperture (NA) of the light delivery optics and the sample. Conventional OCT systems have a trade-off between lateral resolution and depth-of-focus. Clinical OCT systems operating at the 830 nm or 1060 nm center wavelength regions commonly use a beam diameter of ˜1 mm at the cornea, corresponding to a lateral resolution on the order of ˜20 μm at the retina. This resolution is approximately one order of magnitude worse than the theoretical best axial OCT resolution, which is on the order of ˜2 μm. By increasing the probe beam diameter at the cornea, the lateral resolution can be improved, but this approach reduces the depth-of-focus as a trade-off.
For retinal OCT imaging, the cornea and lens act as the imaging objective, so the beam diameter at the pupil determines the numerical aperture and hence the focused spot size at the retina. Since imaging through the entire thickness of the retina and structures of the optic nerve head (ONH) is desirable, conventional retinal OCT configurations have a lateral resolution that is less than what is achievable based on the limiting pupil of the eye. Therefore, an extended depth-of-focus imaging system capable of maintaining high lateral resolution within the layers of interest is important. Methods have been proposed to overcome this axial depth limitation when imaging with high resolution. These methods include mechanical motion of the sample arm, the addition of focus-modulating elements, such as acousto-optic tunable lenses and Axicon lenses, and adaptive optics. Multi-beam systems have also been reported. Computational approaches such as interferometric synthetic aperture microscopy (ISAM) have also been used to correct for defocus in post-processing and provide axial focus extension.